Optical disk unit

Details Optical disk unit Optical disk unit

2000-10-31

2002-08-20

Edun, Muhammad (Department: 2653)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S059110, C369S047500, C369S053100

Reexamination Certificate

active

06438077

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical disk unit, and more particularly, to an optical disk unit capable of recording information to and/or reproducing information from an optical disk recording medium.
2. Description of the Related Art
Recordable optical disks are of two general types, the so-called Write Once and the Erasable. The former includes the so-called Compact Disk Recordable, or CD-R, and the Compact Disk Rewritable, or CD-RW. The CD-R and the CD-RW are provided with a guide, that is, a pregroove. The pregroove undulates slightly in the radial direction of the disk around a central wavelength of 22.05 kHz, and address information called ATIP (Absolute Time In Pregroove) is FSK modulated, stacked and recorded at a maximum displacement of ±1 kHz.
The tracking and focus servo circuits of the optical disk units that record and reproduce information to and from the recordable optical disks described above project light onto the optical disk and detect light reflected back from the optical disk using a plurality of photosensors, generate a tracking error signal using a predetermined set of calculations, and drive a tracking actuator based thereon.
The optical disk units record and reproduce information to and from a CD-R by using the light beam for the read power during reproduction and alternating the light beam between a write power and a read power (the write power being greater than the read power) corresponding to recording signal values 0 and 1. For this reason, during reproduction as a matter of course but also during recording as well, sampling the timing of the reflected light generates a tracking error signal. It should be noted that the write power is greater than the read power.
With the optical disk units that record and reproduce information to and from erasable CD-RW type disks, the light beam power is alternated between a write power and an erase power, (the write power being stronger than the erase power which is stronger than the read power) corresponding to the recording signal values 0, 1.
As a result, the light reflected back from the disk (the light beam power being at read power) is detected and a tracking error signal (if any) is generated. During recording, the light beam power, which is at erase power, is sample held and a tracking error signal generated.
The differential push-pull method is one common tracking control means.
FIG. 1
is a schematic diagram showing three light beam spots used in the differential push-pull tracking method.
The differential push-pull method involves projecting a main light beam spot
2
onto the track n formed by the pregrooves
1
as shown in
FIG. 1
, and, at the same time, offsetting a leading sub beam spot
3
and a trailing sub beam spot
4
from the track n by a predetermined distance in a direction of a width of the pregroove
1
. A reflection of the main light beam spot
2
is detected by photosensors
10
A,
10
B separated along the width direction of the pregroove
1
as shown in
FIG. 2
, a reflection of the leading sub beam spot
3
is detected by photosensors
12
A,
12
B separated along the width direction of the pregroove
1
, and a reflection of the trailing sub beam spot
14
A,
14
B is detected by photosensors
14
A,
14
B separated along the width direction of the pregroove. It should be noted that in
FIG. 1
the light beam spots
2
,
3
and
4
are given reference letters A, B which corresponds to the portions detected by the photosensors
10
A,
10
B,
12
A,
12
B,
14
A,
14
B.
FIG. 2
is a diagram showing the circuit structure of one example of a conventional tracking error detection circuit using the differential push-pull method. In
FIG. 2
, the detection signals of the photosensors
10
A,
10
B are supplied via a sample-and-hold circuit
16
to a noninverted input pin and an inverted input pin of a subtracting circuit
18
, with a differential signal output by the subtracting circuit
18
being supplied to a noninverted input pin of another subtracting circuit
20
.
It should be noted that the sample-and-hold circuit
16
samples the respective detection signals of the photosensors
10
A,
10
B,
12
A,
12
B,
14
A,
14
B at read power and holds at write power.
The respective detection signals of photosensors
12
A,
12
B are supplied via the sample-and-hold circuit
16
to one of the input pins of adding circuits
22
,
24
, and the respective detection signals of photosensors
14
A,
14
B are supplied to the other input pins of the adding circuits
22
,
24
. The adding circuit
22
adds the two signals supplied and supplies the sum to a noninverted input pin of a subtracting circuit
26
, and the adding circuit
24
adds the two signals supplied and supplies the sum to the inverted input pin of the subtracting circuit
26
. In order to carry out the differential push-pull tracking method, the differential signal output from the subtracting circuit
26
is amplified by an amplifier
28
to for example a gain
7
and then supplied to an inverted input pin of the subtracting circuit
20
.
The error signal output by the subtracting circuit
20
is supplied to the adder
30
and either a reproduction offset or a recording offset supplied from the switch
32
is added and the sum output from the pin
34
as a tracking error signal. By controlling the tracking so that the tracking error signal becomes zero, the main light beam spot
2
follows the pregroove
1
, that is, tracking is carried out.
However, the laser beam emitted from the laser diode is stronger or weaker depending on whether it is set to read power or write power, and the axis of the laser beam can be displaced as a result.
FIG. 3
is a diagram for explaining a displacement of an axis of a laser beam emitted from a laser diode. As shown in
FIG. 3
, the laser beam emitted from the laser diode
36
at read power is shown as a straight line, with the laser beam emitted from the laser diode
36
at write power shown as a dotted line, resulting in a displacement of the axis of the beam by an angle &thgr;. When the direction of displacement of the beam axis is in the direction of the width of the pregroove
1
, then during recording as well a tracking error signal is generated at read power level and so the recording region recorded at write power (pits in the case of a CD-R) slips away from the pregroove
1
. As a result, a switch
32
for correcting this displacement is provided, in order to switch between replay offset and recording offset.
Conventionally, the process of producing a disk unit involves actually recording signals on an optical disk, and reproducing that recorded portion so as to detect and maintain a tracking error signal offset portion. However, a difficulty arises in that the recording offset detected at the manufacturing stage is used and remains fixed thereat, whereas changes in temperature and the passage of time result in changes in the characteristics of the laser diode
36
and a consequent change in the amount of offset of the beam axis at recording with respect to that at reproduction. Hence, a tracking error arises and accurate tracking cannot be conducted.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved and useful optical disk unit in which the above-described disadvantage is eliminated.
A further and more specific object of the present invention is to provide an improved and useful optical disk unit in which tracking error can be avoided and correct tracking carried out even if the amount of displacement of the light beam of the laser diode during recording changes.
The above-described objects of the present invention are achieved by an optical disk unit that directs a light beam onto a pregroove on an optical disk and generates a tracking error signal for tracking control based on a detection signal generated from a reflection of the light beam, the optical disk unit comprising a recording offset generator that acquires a differential between a tracking error component ge

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